The present invention relates to a gas/liquid contacting apparatus for the exchange of matter between a gas phase and a liquid phase and more in particular, to a column in which gas and liquid are caused to flow countercurrently.
Gas/liquid contacting columns are widely applied in the industry for a large variety of processing operations, such as for example, distillation, absorption, stripping and evaporation. The columns which are most commonly used for gas/liquid contact are the so-called plate columns and the so-called packed columns. Generally the plate columns are of the cross-flow type, wherein the columns are provided with trays having openings for ascending gas and one or more downcomers for descending liquid. Packed columns for gas/liquid contact, being used extensively for absorption operations, are columns filled with randomly oriented packing material or with structured packing material. The total open area available for ascending gas is in packed columns very high compared to plate columns. It will therefore be understood that packed columns can be operated at far higher gas loads than plate columns. A disadvantage of packed columns is however, the operating range for the liquid load. Low liquid rates lead to incomplete wetting of the column packings thus a decreasing contacting efficiency. On the other hand, at higher liquid rates the effective open cross sectional area available for the passage of gas becomes smaller because of the presence of liquid, and part of the momentum of the gas stream is used to support an increasing quantity of liquid in the column, resulting in an increase of the liquid hold-up and unstable operation.
Plate columns of the cross flow type can be more readily designed for high liquid loads than packed columns. The minimum liquid load which is admissible depends on the minimum liquid velocity through the downcomer liquid discharge openings or the minimum load required to keep sufficient liquid on the contact trays. The maximum allowable liquid load can be increased by increasing the length of the downcomer weirs on the trays, for example, the length of the downcomers and/or the number of downcomers may be enlarged, all of which measures mean an increase of the total tray area occupied by downcomers. Increase of the total downcomer area, however, results in a decrease of the total free area of the trays, i.e., those parts of the trays above which gas and liquid are contacted with one another and which trays are provided with apertures for ascending gas. Reduction of the total free area of a contact tray, however, results in a reduction of the total area available for the passage of ascending gas. At a given gas load, reduction of the total free area of the contact trays constitutes increase of the gas velocity upon passage through the contact tray apertures, which in its turn might give rise to flooding of the column. Flooding of the column is the phenomenon that occurs when the spaces between adjacent contact trays are completely filled with a froth of liquid and gas. From the above it will be clear that although the liquid capacity of plate columns may be relatively easily increased by enlargement of the downcomer area, such an increase will adversely affect the maximum gas load. With respect to this statement it should be understood that the spacing and the size of the apertures in contact trays for ascending gas can only be varied within rather narrow limits. Very small apertures may lead to punching difficulties and possibilities of fouling in dirty service, while large holes may lead to weeping and poor dispersion. The spacing of the apertures usually ranges from 2.5 to 4 times the diameter of the apertures. Closer spacings lead to excessive weeping and wider spacings lead to excessive pressure drop and to entrainment due to high apertures velocities.